1. Field of the Invention
This invention pertains to friction stir welding and more particularly to a backing plate and a friction stir welding probe for making L- and T-type filleted corner joints.
2. Background of the Invention
Friction stir welding is a relatively new welding technique discovered in the mid-1990s that was developed primarily for welding aluminum and soft aluminum alloys that were difficult to weld using traditional welding techniques. The technique uses a rotating shouldered cylindrical tool with a projecting probe (also referred to as a pin) to generate heat in the workpiece. The mechanical friction of the rotating tool contacts the workpiece and plasticizes (softens) the metal as it is plunged into the bondline. At this stage, there is a substantial amount of plasticized metal in a column about the rotating pin beneath the shoulder of the cylinder portion of the tool. The tool is then moved along the bondline relative to the workpiece. As the pin rotates and moves in a transverse direction, the metal is plasticized at the front of the pin and extruded to the back of the pin while undergoing a mechanical stirring and forging action imparted by the pin surface profile and confined from above by the pressure exerted on the material by the shoulder of the cylindrical tool. The plasticized metal is transferred from the front of the pin around the periphery of both sides of the pin and subsequently reconstituted at the back of the pin to produce the weld. The rotational speed and translational travel speed are controlled to maintain a plasticized metal state.
The friction stir weld tool is formed as a cylindrical piece with a shoulder face that meets a probe that projects from the shoulder face at a right angle (U.S. Pat. Nos. 5,460,317 and 6,029,879). In some instances, the probe actually moves in a perpendicular direction in an aperture formed in the face of the shoulder (U.S. Pat. Nos. 5,697,544; 5,718,366; and 5,893,507). The face of the shoulder can be formed with an upward dome that is perpendicular to the probe (U.S. Pat. Nos. 5,611,479; 5,697,544; and 6,053,391). The dome region and an unobstructed dome (shoulder face)/probe interface are considered essential for the proper frictional heating of the workpiece material. The dome region constrains plasticized material for consolidation at the trailing edge of the friction stir tool rather than permitting it to extrude out from under the sides of the tool. The length of the probe is usually designed to be slightly shorter than the thickness of the materials being welded. The bottom (distal end) of the probe is also normally made with a smooth radius. The features on the typically smooth distal end of the probe are usually not considered critical other than to assure that a certain clearance distance from the bottom of the work pieces being welded are maintained for any joint configuration.
Another factor influencing friction stir weld is the configuration of the backing or support plate. The support plate is the xe2x80x9ctoolxe2x80x9d on which the work pieces rest during welding. For butt welds, the work pieces sit on a flat featureless plate that only restricts movement of the plasticized work piece material during welding. For L- and T-type corner joints, backing plates butt directly up against both the horizontal and vertical work pieces so that no material can extrude away from the joint. Welding with this type of joint configuration with the backing plates firmly against the horizontal and vertical members and a standard friction stir welding tool (tapered or straight probe) often produces a crevice at the point where the horizontal and vertical work pieces contact each other because the probe has not properly mixed the plasticized materials. For various applications, this can act as either a notch when the joint is loaded in fatigue or as a source of corrosion by harboring detrimental work piece compositions. To remedy these situations, either a subsequent machining operation is needed to smooth the corner or an additional welding process (such as gas metal arc) is needed to fill the corner in a sealing-type operation.
Accordingly, it is an object of the present invention to provide a friction stir welding setup for corner welds that avoids volumetric defects present after welding.
It is an object of the present invention to avoid subsequent machining operations to smooth the corners of L- and T-type friction stir welds.
It is another object of the present invention to provide a method of friction stir welding L- and T-type corner joints that avoids the production of crevices at the point where the two work pieces meet.
It is another object of the present invention to provide a method of friction stir welding that avoids subsequent additional welding processes to fill the corner of L- and T-type welds.
It is an object of the present invention to produce a filleted L- and T-type corner weld using the friction stir welding process.
It is an object of the present invention to produce a corner support (backing) plate that promotes the production of a filleted L- and T-type corner weld using the friction stir welding process.
It is an object of the present invention to provide a friction stir weld tool that thoroughly mixes plasticized work piece material near the distal end of the probe.
It is an object of the present invention to provide a friction stir weld tool that forms a well mixed plasticized work piece material in the region of a fillet of a L- and T-type corner weld.
In order to meet these objects, the present invention of a friction stir weld tool features a friction stir welding setup for corner welds that comprises a corner support plate having: 1) a horizontal portion, 2) an angled portion, and 3) a radiused outer corner. A first work piece is secured to the angled portion of the corner support plate while a second work piece is secured to the horizontal corner support plate and in proximity with said first work piece to form a corner void with the radiused outer corner of the corner support plate. A friction stir weld tool is used to join the first work piece to the second work piece in a region where the first work piece and said second work piece are in proximity with each other and to fill the corner void with plasticized first work piece and second work piece material to formed a filleted corner weld.
For an L-type weld, the first work piece extends above the horizontal portion of corner support plate by an amount equal to the thickness of the second work piece and an end of the second work piece butts against the face of the first work piece that extends above the horizontal portion of the corner support plate.
For a T-type weld, the end of the first work piece is substantially flush with the upper surface of the horizontal portion of the corner support plate and the underside of the second work piece butts against the end of the first work piece that is flush with the upper surface of the horizontal portion of the corner support plate. Although in some cases it is desirable to fillet only one side of the T-type weld, typically a second corner support plate with a radius outer corner is used in order to form a fillet on both sides of the T-type weld. The second work piece may be a single unitary piece or made up of two work piece sections in end-to-end proximity relation with each other, i.e., in butt weld relation, and aligned such that the ends are substantially centered over the end of the first work piece.
Preferably for T-weld type welds, the friction stir weld tool tip extends into the end of the first work piece and has a rounded distal end. To ensure complete mixing of plasticized work piece material and filling of the corner void to produce a quality fillet, the friction stir weld tool has a curved tip region that conforms generally to the radius of the outer corner of the corner support plate.
Generally for either L- or T-type welds, the friction stir weld tool has disrupting features such as notches or grooves formed in the distal end of the tool to ensure mixing of plasticized material below the distal end of the tool and force plasticized material into the corner void to form the weld fillet. Two rectangular grooves that are formed in a rounded distal end of the tool probe and cross at right angles are especially effective in carrying out these objectives.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail and illustrated in the accompanying drawings. It is contemplated that variations in size, structural features and probe designs may appear, to a person skilled in the art, without departing from the scope of or sacrificing any of the advantages of the invention.